Peter Geoghegan <> writes:
> On Fri, Sep 30, 2016 at 4:46 PM, Robert Haas <> wrote:
>> The fact that we have hash indexes already and cannot remove them
>> because too much other code depends on hash opclasses is also, in my
>> opinion, a sufficiently good reason to pursue improving them.

> I think that Andres was suggesting that hash index opclasses would be
> usable with hash-over-btree, so you might still not end up with the
> wart of having hash opclasses without hash indexes (an idea that has
> been proposed and rejected at least once before now). Andres?

That's an interesting point.  If we were to flat-out replace the hash AM
with a hash-over-btree AM, the existing hash opclasses would just migrate
to that unchanged.  But if someone wanted to add hash-over-btree alongside
the hash AM, it would be necessary to clone all those opclass entries,
or else find a way for the two AMs to share pg_opclass etc entries.
Either one of those is kind of annoying.  (Although if we did do the work
of implementing the latter, it might come in handy in future; you could
certainly imagine that there will be cases like a next-generation GIST AM
wanting to reuse the opclasses of existing GIST, say.)

But having said that, I remain opposed to removing the hash AM.
If someone wants to implement hash-over-btree, that's cool with me,
but I'd much rather put it in beside plain hash and let them duke
it out in the field.

                        regards, tom lane

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